Possible Approaches
If the low-vision patient cannot resolve the retinal image, despite the fact that it is optimally focussed onto the retina by their refractive correction, then it is necessary for it to be made larger. The intention is that the larger retinal image is sufficiently large for the patient to recognise, despite the ocular pathology impairing their visual performance.
From Fig. 5.1 , it can be seen that the angles subtended at the nodal point of the eye by rays of light from both the object and the image are the same: that is, the ray of light from the top of the object passes straight through the nodal point without deviation, and forms the top of the image. Fig. 5.1A represents the situation before any magnifying device is introduced: it can be seen that the retinal image size is proportional to the angle subtended at the nodal point, and if there is to be an increase in the retinal image size, then this ray of light from the object must form a larger angle at the nodal point of the eye.
There are some important general points to be made concerning magnification as it relates to visual impairment (VI):
- 1.
It is relative: it is the ratio comparing the situation before and after some change in the viewing environment, or perhaps with and without some optical or electronic appliance. Because magnification is relative, it cannot have a value of 0. Magnification may therefore be <1, in which case the retinal image is smaller, or it is >1, which is the situation required to improve acuity. (Note that the use of optical and electronic devices to minify images [magnification <1×], will be discussed in Chapter 12 ).
- 2.
The apparent magnification achieved by an optical magnifier may not directly relate to the magnification formula. For example, an observer looking through a magnifier lens might compare the image as seen outside and through the magnifier, but this is not the same as the change in retinal image size for the observer with and without the magnifier as expressed in the magnification formula. By contrast, for an electronic magnifier, the apparent magnification experienced by comparing the image outside and through the device relates well to the magnification formula.
- 3.
The degree of magnification varies considerably for the different approaches to be described here: likely values range from 1.5× to 5× for simple strategies like changing object size and distance; 1.5× to 20× for optical devices; and 3× to 100× for electronic devices.
- 4.
To put this degree of magnification into context, it can be compared to spectacle magnification. Spectacle magnification is the effect of plus and minus lenses used to correct ametropia: minus lenses make images smaller, whilst plus lenses make them bigger. This does affect retinal image size (the high myope in spectacles may have slightly poorer visual acuity (VA) than when wearing contact lenses) and also affects cosmetic appearance—the observer sees the spectacle wearer’s eyes as smaller or larger. However, the amount of spectacle magnification is very small in comparison to magnifying devices—about 1% per dioptre. Putting this into terms of retinal image size increase, 4% spectacle magnification would be 1.04× magnification, for example: or to put it another way, 3× magnification would be 300%.
In mathematical terms:
magnification(M)=new retinal image sizeoriginal retinal image size
Fig. 5.1A shows the situation ‘before’ magnification, with the retinal image size proportional to θ , the angle subtended by the object at the nodal point of the eye ( N E ). Figs 5.1B to 5.1E show the situation ‘after’ the different forms of magnification have been used, with the magnified retinal image size proportional to the new angle θ’ subtended at the nodal point of the eye.
Therefore,
magnification=θ’θwhich, for small angles,≈tanθ’tanθ
There are four ways in which magnification can be achieved, which are summarised here and explored in detail in the forthcoming chapters.
Relative Size Magnification (Increasing Object Size)
This is illustrated schematically in Fig. 5.1B .
M=tanθ’tanθ=h2d1h1d1=h2×d1d1×h1
M=h2h1=new object sizeoriginal object size
The most common example of the use of this type of magnification is large print either commercially available (such as for books or music), in correspondence (such as a large print utility bill), locally printed (such as handouts for a student with VI in school) or on a screen (such as increasing font size to read messages on a smartphone). The magnification achieved can be determined very simply by a direct measurement of the size of the letter in the large print sample compared to an equivalent letter in a sample of the original text. In this case, we might not actually have the ‘original’ object to carry out the measurement, so it may be more accurate to say that the magnification is relative to a habitual or expected size for normal print. This form of magnification is usually limited to about 2.5× on paper because of physical limits to the size of book or page which can be obtained practically, but much larger amounts can be achieved on a screen ( Fig. 5.2 ) (see Chapter 6 ). This form of magnification could also be used for a distance task: for example, obtaining a larger screen television. In this case, practicality (and cost) will limit the amount of magnification available.
Relative Distance Magnification (Decreasing Viewing Distance)
M=tanθ’tanθ=h1d3h1d1=h1×d1d3×h1
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